Field of the Invention
The present invention relates to a gas concentration sensor for measuring the concentration of combustible gas, such as vaporized fuel, contained in, for example, intake air to be supplied into an intake pipe of, for example, an internal combustion engine, or for measuring the concentration of a gas component in fuel gas of a fuel cell or in exhaust gas. 2. Description of the Related Art
Conventionally, a fuel supply system for supply of fuel from a fuel tank to an engine includes a first supply system which functions in the following manner. Fuel is pumped from the fuel tank by means of a fuel pump and is then sent to an injector through a fuel pipe.
The fuel supply system further includes a second supply system which functions in the following manner. Vaporized fuel generated within the fuel tank is temporarily adsorbed by a canister. Accumulated vaporized fuel is purged from the canister and is sent as purge gas to an intake pipe.
Accordingly, in addition to fuel injected from the injector, vaporized fuel, such as purge gas, is burned within a cylinder of the engine (hereinafter vaporized fuel is referred to merely as purge gas).
When, as a result of supply to the engine of purge gas in addition to injected fuel, an air-fuel ratio deviates from a theoretical value, the purification capability of a catalyst with respect to CO, HC, and NOx lowers considerably. As a result, CO, HC, and NOx contents of exhaust gas increase.
Accordingly, in order to use purge gas as a portion of main fuel for combustion, for example, at engine start-up, particularly when the catalyst is inactive, optimum control of purge gas supply through highly accurate measurement of purge gas concentration is very important.
A sensor for measuring purge gas concentration may utilize, for example, ultrasonic waves (ultrasonic sensor). Such an ultrasonic sensor has been developed, but a satisfactory ultrasonic sensor has not yet been developed.
Some ultrasonic sensors of this kind have an ultrasonic element for transmitting and receiving an ultrasonic wave having a modulation point. On the basis of the difference in time between a modulation point present in a transmitted wave and that in a received wave (i.e., on the basis of propagation time of an ultrasonic wave), purge gas concentration is determined. An ultrasonic wave which the ultrasonic element receives is actually a combined wave of a direct wave and an indirect wave. The direct wave is a component having highest sound pressure and propagating along a shortest path, and the indirect wave is a component having relatively low sound pressure and propagating along a longer path than that of the direct wave.
The indirect wave is slightly delayed in propagation with respect to the direct wave and is combined with the direct wave at a portion located in the vicinity of a modulation point of the direct wave. This causes difficulty in detecting the modulation point of the direct wave, resulting in a failure to accurately measure the propagation time of the direct wave.
Accordingly, accurate control of purge gas concentration on the basis of a measured purge gas concentration becomes considerably difficult.
In recent years, development of a fuel cell for use as a clean automobile power source has been carried out intensively. Fuel cells include a molten-carbonate fuel cell and a phosphate fuel cell. Especially, a polyelectrolyte fuel cell (PEFC) is of particular interest because of various advantages, such as easy starting and stopping, high output density, compactness, and light weight.
A polyelectrolyte fuel cell employs hydrogen as fuel. Hydrogen serving as fuel is conventionally produced by means of a reformer, in the form of a gas reformed from methanol. In order to efficiently generate power, measurement of hydrogen concentration in the reformed gas is very important. Presupposing use within combustible gas, a gas concentration sensor for measuring the hydrogen concentration desirably has a low working temperature. Japanese Patent Publication (kokoku) No. 7-31153 discloses an example of such a sensor; specifically, a sensor which employs a proton conductor film. However, the disclosed sensor involves a certain degree of heating. Also, a current-measurement-type sensor which employs NAPHYON as an electrolyte is proposed. However, this sensor has a problem in that an electrode is poisoned by CO, which is generated in a large amount during start-up, and that humidity dependency is high.
In order to detect gas concentration without involving heating and to diminish poisoning by a miscellaneous gas component or humidity dependency, a technique which utilizes the above-mentioned ultrasonic sensor used in an engine is proposed. However, when a gas component having a low molecular weight, such as hydrogen gas, is to be detected, the speed of sound becomes considerably high. As a result, the indirect wave is highly likely to overlap the direct wave, causing difficulty in detecting a modulation point. Therefore, measurement of propagation time becomes considerably difficult.
In the case of receiving such a high-speed sound wave, even a controller's switching noise, which is generated at the time of transmission of an ultrasonic wave, and reverberations overlap a received wave reflected from a reflection surface, thus disturbing wave reception.
There can also be a problem with an ultrasonic gas concentration sensor that, when the flow velocity of intake air is high, with a resultant disturbance of flow, an ultrasonic wave transmitted into the intake air suffers unstable amplitude attenuation, resulting in a failure to receive an ultrasonic wave of stable amplitude. As a result, propagation time cannot be determined accurately.